An engine timing drive typically includes an endless timing chain, such as a roller chain, engaged with a crankshaft sprocket and one or more camshaft sprockets. The timing drive usually also includes a chain guide mechanism for guiding the timing chain. The mechanism includes a pivoted guide lever having a shoe in sliding engagement with the chain, and a chain tensioner having a fixed tensioner housing, and a plunger protruding therefrom for biasing the lever against the chain in order to maintain adequate tension in the chain.
Oil is supplied under pressure to a high pressure oil chamber formed by the plunger and the plunger-accommodating hole in the tensioner housing. The oil flows out of the high pressure oil chamber through a restricted space between the plunger and the wall of the plunger-accommodating hole when the chain applies a force to lever, and through the lever to the plunger, urging the plunger in the retracting direction. Because of space for flow of oil out of the high pressure oil chamber is restricted, retraction of the plunger is controlled by hydraulic damping.
On engine start-up, however, until oil pressure builds up in the high pressure oil chamber, adequate damping does not take place, the plunger can recede excessively, and rattling of the chain and of the tensioning mechanism can occur. To address the problem of rattling due to excessive receding movement of the plunger during engine start-up, various devices have been proposed to restrict retracting movement of the plunger before pressure builds up in the oil in the high pressure oil chamber. In one such device, described in Japanese Patent No. 3929680, a ratchet is composed of a plurality of grooves formed on the outer periphery of the plunger and an engaging member provided on the tensioner housing. The engaging member cooperates with alternating concave and convex surfaces formed by the grooves to limit retraction of the plunger even when the supply of oil under pressure to the high pressure oil chamber of the tensioner is inadequate.
A problem with the above-described ratchet mechanism is that, because it also restricts retracting movement of the plunger caused by excessive tension in the chain resulting from changes in engine temperature and other causes, the chain can operate under excessive tension resulting in increased noise and damage to the chain.
The ratchet mechanism can be designed to allow a predetermined amount of backlash corresponding to an assumed maximum value of the retracting movement of the plunger caused by the excessive tension in the chain. However, as the allowed backlash is increased to accommodate increases in chain tension, a greater amount of rattling can occur on engine start-up.
If the engaging member of the ratchet mechanism is provided with a cam to alleviate the above problem, the cam will have a protruding portion on the outside of the tensioner housing, making the tensioner structure and shape complicated, and causing difficulties in the assembly, installation and maintenance of the tensioner.
Another problem is that it has been necessary to design the ratchet mechanism so that it has different characteristics depending on its conditions of use, and consequently, its production cost is increased.